RFC1773 - Experience with the BGP-4 protocol

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　　Network Working Group P. Traina
Request for Comments: 1773 cisco Systems
Obsoletes: 1656 March 1995
Category: Informational

Experience with the BGP-4 protocol

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Introduction

The purpose of this memo is to document how the requirements for
advancing a routing protocol to Draft Standard have been satisfied by
Border Gateway Protocol version 4 (BGP-4). This report documents
experience with BGP. This is the second of two reports on the BGP
protocol. As required by the Internet Architecure Board (IAB) and
the Internet Engineering Steering Group (IESG), the first report will
present a performance analysis of the BGP protocol.

The remaining sections of this memo document how BGP satisfies
General Requirements specified in Section 3.0, as well as
Requirements for Draft Standard specified in Section 5.0 of the
"Internet Routing Protocol Standardization Criteria" document [1].

This report is based on the initial work of Peter Lothberg (Ebone),
Andrew Partan (Alternet), and several others. Details of their work
were presented at the Twenty-fifth IETF meeting and are available
from the IETF proceedings.

Please send comments to iwg@ans.net.

Acknowledgments

The BGP protocol has been developed by the IDR (formerly BGP) Working
Group of the Internet Engineering Task Force. I would like to
express deepest thanks to Yakov Rekhter and Sue Hares, co-chairs of
the IDR working group. I'd also like to explicitly thank Yakov
Rekhter and Tony Li for the review of this document as well as
constructive and valuable comments.

Documentation

BGP is an inter-autonomous system routing protocol designed for
TCP/IP internets. Version 1 of the BGP protocol was published in RFC
1105. Since then BGP Versions 2, 3, and 4 have been developed.
Version 2 was documented in RFC1163. Version 3 is documented in RFC
1267. The changes between versions 1, 2 and 3 are explained in
Appendix 2 of [2]. All of the functionality that was present in the
previous versions is present in version 4.

BGP version 2 removed from the protocol the concept of "up", "down",
and "horizontal" relations between autonomous systems that were
present in version 1. BGP version 2 introduced the concept of path
attributes. In addition, BGP version 2 clarified parts of the
protocol that were "under-specified".

BGP version 3 lifted some of the restrictions on the use of the
NEXT_HOP path attribute, and added the BGP Identifier field to the
BGP OPEN message. It also clarifies the procedure for distributing
BGP routes between the BGP speakers within an autonomous system.

BGP version 4 redefines the (previously class-based) network layer
reachability portion of the updates to specify prefixes of arbitrary
length in order to represent multiple classful networks in a single
entry as discussed in [5]. BGP version 4 has also modified the AS-
PATH attribute so that sets of autonomous systems, as well as
individual ASs may be described. In addition, BGP version for has
redescribed the INTER-AS METRIC attribute as the MULTI-EXIT
DISCRIMINATOR and added new LOCAL-PREFERENCE and AGGREGATOR
attributes.

Possible applications of BGP in the Internet are documented in [3].

The BGP protocol was developed by the IDR Working Group of the
Internet Engineering Task Force. This Working Group has a mailing
list, iwg@ans.net, where discussions of protocol features and
operation are held. The IDR Working Group meets regularly during the
quarterly Internet Engineering Task Force conferences. Reports of
these meetings are published in the IETF's Proceedings.

MIB

A BGP-4 Management Information Base has been published [4]. The MIB
was written by Steve Willis (Wellfleet), John Burruss (Wellfleet),
and John Chu (IBM).

Apart from a few system variables, the BGP MIB is broken into two
tables: the BGP Peer Table and the BGP Received Path Attribute Table.

The Peer Table reflects information about BGP peer connections, such
as their state and current activity. The Received Path Attribute
Table contains all attributes received from all peers before local
routing policy has been applied. The actual attributes used in
determining a route are a subset of the received attribute table.

Security Considerations

BGP provides flexible and extendible mechanism for authentication and
security. The mechanism allows to support schemes with various
degree of complexity. All BGP sessions are authenticated based on
the BGP Identifier of a peer. In addition, all BGP sessions are
authenticated based on the autonomous system number advertised by a
peer. As part of the BGP authentication mechanism, the protocol
allows to carry encrypted digital signature in every BGP message.
All authentication failures result in sending the NOTIFICATION
messages and immediate termination of the BGP connection.

Since BGP runs over TCP and IP, BGP's authentication scheme may be
augmented by any authentication or security mechanism provided by
either TCP or IP.

However, since BGP runs over TCP and IP, BGP is vulnerable to the
same denial of service or authentication attacks that are present in
any other TCP based protocol.

Implementations

There are multiple independent interoperable implementations of BGP
currently available. This section gives a brief overview of the
implementations that are currently used in the operational Internet.
They are:

- cisco Systems
- gated consortium
- 3COM
- Bay Networks (Wellfleet)
- Proteon

To facilitate efficient BGP implementations, and avoid commonly made
mistakes, the implementation experience with BGP-4 in with cisco's
implementation was documented as part of RFC1656 [4].

Implementors are strongly encouraged to follow the implementation
suggestions outlined in that document and in the appendix of [2].

Experience with implementing BGP-4 showed that the protocol is
relatively simple to implement. On the average BGP-4 implementation
takes about 2 man/months effort, not including any restructuring that
may be needed to support CIDR.

Note that, as required by the IAB/IESG for Draft Standard status,
there are multiple interoperable completely independent
implementations.

Operational experience

This section discusses operational experience with BGP and BGP-4.

BGP has been used in the production environment since 1989, BGP-4
since 1993. This use involves at least two of the implementations
listed above. Production use of BGP includes utilization of all
significant features of the protocol. The present production
environment, where BGP is used as the inter-autonomous system routing
protocol, is highly heterogeneous. In terms of the link bandwidth it
varies from 28 Kbits/sec to 150 Mbits/sec. In terms of the actual
routes that run BGP it ranges from a relatively slow performance
PC/RT to a very high performance RISC based CPUs, and includes both
the special purpose routers and the general purpose workstations
running UNIX.

In terms of the actual topologies it varies from a very sparse
(spanning tree of ICM) to a quite dense (NSFNET backbone).

At the time of this writing BGP-4 is used as an inter-autonomous
system routing protocol between ALL significant autonomous systems,
including, but by all means not limited to: Alternet, ANS, Ebone,
ICM, IIJ, MCI, NSFNET, and Sprint. The smallest know backbone
consists of one router, whereas the largest contains nearly 90 BGP
speakers. All together, there are several hundred known BGP speaking
routers.

BGP is used both for the exchange of routing information between a
transit and a stub autonomous system, and for the exchange of routing
information between multiple transit autonomous systems. There is no
distinction between sites historically considered backbones vs
"regional" networks.

Within most transit networks, BGP is used as the exclusive carrier of
the exterior routing information. At the time of this writing within
a few sites use BGP in conjunction with an interior routing protocol
to carry exterior routing information.

The full set of exterior routes that is carried by BGP is well over
20,000 aggregate entries representing several times that number of
connected networks.

Operational experience described above involved multi-vendor
deployment (cisco, and "gated").

Specific details of the operational experience with BGP in Alternet,
ICM and Ebone were presented at the Twenty-fifth IETF meeting
(Toronto, Canada) by Peter Lothberg (Ebone), Andrew Partan (Alternet)
and Paul Traina (cisco).

Operational experience with BGP exercised all basic features of the
protocol, including authentication, routing loop suppression and the
new features of BGP-4, enhanced metrics and route aggregation.

Bandwidth consumed by BGP has been measured at the interconnection
points between CA*Net and T1 NSFNET Backbone. The results of these
measurements were presented by Dennis Ferguson during the Twenty-
first IETF, and are available from the IETF Proceedings. These
results showed clear superiority of BGP as compared with EGP in the
area of bandwidth consumed by the protocol. Observations on the
CA*Net by Dennis Ferguson, and on the T1 NSFNET Backbone by Susan
Hares confirmed clear superiority of the BGP protocol family as
compared with EGP in the area of CPU requirements.

Migration to BGP version 4

On multiple occasions some members of IETF expressed concern about
the migration path from classful protocols to classless protocols
such as BGP-4.

BGP-4 was rushed into production use on the Internet because of the
exponential growth of routing tables and the increase of memory and
CPU utilization required by BGP. As such, migration issues that
normally would have stalled deployment were cast aside in favor of
pragmatic and intelligent deployment of BGP-4 by network operators.

There was much discussion about creating "route exploders" which
would enumerate individual class-based networks of CIDR allocations
to BGP-3 speaking routers, however a cursory examination showed that
this would vastly hasten the requirement for more CPU and memory
resources for these older implementations. There would be no way
internal to BGP to differentiate between known used networks and the
unused portions of the CIDR allocation.

The migration path chosen by the majority of the operators was known
as "CIDR, default, or die!"

To test BGP-4 operation, a virtual "shadow" Internet was created by
linking Alternet, Ebone, ICM, and cisco over GRE based tunnels.
Experimentation was done with actual live routing information by
establishing BGP version 3 connections with the production networks
at those sites. This allowed extensive regression testing before
deploying BGP-4 on production equipment.

After testing on the shadow network, BGP-4 implementations were
deployed on the production equipment at those sites. BGP-4 capable
routers negotiated BGP-4 connections and interoperated with other
sites by speaking BGP-3. Several test aggregate routes were injected
into this network in addition to class-based networks for
compatibility with BGP-3 speakers.

At this point, the shadow-Internet was re-chartered as an
"operational experience" network. tunnel connections were
established with most major transit service operators so that
operators could gain some understanding of how the introduction of
aggregate networks would affect routing.

After being satisfied with the initial deployment of BGP-4, a number
of sites chose to withdraw their class-based advertisements and rely
only on their CIDR aggregate advertisements. This provided
motivation for transit providers who had not migrated to either do
so, accept a default route, or lose connectivity to several popular
destinations.

Metrics

BGP version 4 re-defined the old INTER-AS metric as a MULTI-EXIT-
DISCRIMINATOR. This value may be used in the tie breaking process
when selecting a preferred path to a given address space. The MED is
meant to only be used when comparing paths received from different
external peers in the same AS to indicate the preference of the
originating AS.

The MED was purposely designed to be a "weak" metric that would only
be used late in the best-path decision process. The BGP working
group was concerned that any metric specified by a remote operator
would only affect routing in a local AS if no other preference was
specified. A paramount goal of the design of the MED was insure that
peers could not "shed" or "absorb" traffic for networks that they
advertise.

The LOCAL-PREFERENCE attribute was added so a local operator could
easily configure a policy that overrode the standard best path
determination mechanism without configuring local preference on each
router.

One shortcoming in the BGP4 specification was a suggestion for a
default value of LOCAL-PREF to be assumed if none was provided.
Defaults of 0 or the maximum value each have range limitations, so a
common default would aid in the interoperation of multi-vendor
routers in the same AS (since LOCAL-PREF is a local administration
knob, there is no interoperability drawback across AS boundaries).

Another area where more exploration is required is a method whereby
an originating AS may influence the best path selection process. For
example, a dual-connected site may select one AS as a primary transit
service provider and have one as a backup.

/---- transit B ----\
end-customer transit A----
\---- transit C ----/

In a topology where the two transit service providers connect to a
third provider, the real decision is performed by the third provider
and there is no mechanism for indicating a preference should the
third provider wish to respect that preference.

A general purpose suggestion that has been brought up is the
possibility of carrying an optional vector corresponding to the AS-
PATH where each transit AS may indicate a preference value for a
given route. Cooperating ASs may then chose traffic based upon
comparison of "interesting" portions of this vector according to
routing policy.

While protecting a given ASs routing policy is of paramount concern,
avoiding extensive hand configuration of routing policies needs to be
examined more carefully in future BGP-like protocols.

Internal BGP in large autonomous systems

While not strictly a protocol issue, one other concern has been
raised by network operators who need to maintain autonomous systems
with a large number of peers. Each speaker peering with an external
router is responsible for propagating reachability and path
information to all other transit and border routers within that AS.
This is typically done by establishing internal BGP connections to
all transit and border routers in the local AS.

In a large AS, this leads to an n^2 mesh of TCP connections and some
method of configuring and maintaining those connections. BGP does
not specify how this information is to be propagated, so
alternatives, such as injecting BGP attribute information into the
local IGP have been suggested. Also, there is effort underway to
develop internal BGP "route reflectors" or a reliable multicast

transport of IBGP information which would reduce configuration,
memory and CPU requirements of conveying information to all other
internal BGP peers.

Internet Dynamics

As discussed in [7], the driving force in CPU and bandwidth
utilization is the dynamic nature of routing in the Internet. As the
net has grown, the number of changes per second has increased. We
automatically get some level of damping when more specific NLRI is
aggregated into larger blocks, however this isn't sufficient. In
Appendix 6 of [2] are descriptions of dampening techniques that
should be applied to advertisements. In future specifications of
BGP-like protocols, damping methods should be considered for
mandatory inclusion in compliant implementations.

Acknowledgments

The BGP-4 protocol has been developed by the IDR/BGP Working Group of
the Internet Engineering Task Force. I would like to express thanks
to Yakov Rekhter for providing RFC1266. I'd also like to explicitly
thank Yakov Rekhter and Tony Li for their review of this document as
well as their constructive and valuable comments.

Author's Address

Paul Traina
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134

EMail: pst@cisco.com

References

[1] Hinden, R., "Internet Routing Protocol Standardization Criteria",
RFC1264, BBN, October 1991.

[2] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC1771, T.J. Watson Research Center, IBM Corp., cisco Systems,
March 1995.

[3] Rekhter, Y., and P. Gross, Editors, "Application of the Border
Gateway Protocol in the Internet", RFC1772, T.J. Watson Research
Center, IBM Corp., MCI, March 1995.

[4] Willis, S., Burruss, J., and J. Chu, "Definitions of Managed
Objects for the Fourth Version of the Border Gateway Protocol
(BGP-4) using SMIv2", RFC1657, Wellfleet Communications Inc.,
IBM Corp., July 1994.

[5] Fuller V., Li. T., Yu J., and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, BARRNet, cisco, MERIT, OARnet, September
1993.

[6] Traina P., "BGP-4 Protocol Document Roadmap and Implementation
Experience", RFC1656, cisco Systems, July 1994.

[7] Traina P., "BGP Version 4 Protocol Analysis", RFC1774, cisco
Systems, March 1995.

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